The accident, the cause of which remains unclear, occurred near Kavar, a town about 1,000 kilometres (600 miles) from the capital, Tehran.
“Unfortunately, 21 deaths have been recorded,” Kavar Hospital director Mohsen Afrasiabi told state television, adding that 29 people were injured.
Iranian media showed images of a coach lying on its side on a mountain road.
Iran has a poor road safety record, with nearly 20,000 deaths from traffic accidents in the 12 months to March.
